The field of the invention relates to sensors etched from silicon for sensing an applied force. In a particular aspect of the invention, the sensor is utilized as an accelerometer for sensing a force of acceleration. In another aspect, the invention relates to methods for fabricating such accelerometers by etching silicon substrates.
Accelerometers are known having a cantilever which deflects in response to a force of acceleration. Conventional electronic sensing circuitry, such as resistive strain gages attached to the cantilever, provide an electrical measurement related to the acceleration force. It is also known to form the cantilevers of accelerometers by etching silicon substrates thereby achieving the advantage of batch processing techniques known in the semiconductor industry. For example, U.S. Pat. No. 4,653,326 issued to Danel et al discloses etching a cantilever from a silicon substrate which deflects in response to a force component applied in only one direction, parallel to the substrate in this example. Capacitive plates coupled to the cantilever provide an electrical measurement proportional to the applied force.
A problem with these types of accelerometers is that only one force component is detectable. In applications which require detecting all three components of an acceleration force, three separate sensors are needed thereby adding complexity.
U.S. Pat. No. 4,342,227 issued to Petersen et al discloses an accelerometer fabricated by first etching V-shaped grooves, having sidewalls lying in the {111} crystalline planes, into the planar surface of a silicon substrate. Two sets of grooves at right angles are formed and capacitor electrodes defused into the {111} walls. An epitaxial layer is grown over the grooved substrate and an insulation layer grown over the epitaxial layer. A conductive layer is then deposited on the insulation layer to define an electrode. The substrate is again subjected to anisotropic etching for cutting the epitaxial layer from under the insulating layer thereby defining a cantilever beam suspended over the V groove. Since two perpendicular cantilevers are formed, two of the three acceleration force components may be detected. A difficulty appears to be in detecting a third force component normal to the substrate. Allegedly, the cantilevers move in both lateral and up/down directions. Discriminating electronic circuitry and associated calculations are then used in an effort to detect the normal force component. Applicant contends that there is a complex relationship between the electrical measurement and normal force component thereby resulting in a complex and possibly inaccurate calculation of the normal component. In addition, there are apparent fabrication complexities in suspending the cantilevers over the V grooves.
The inventor herein has recognized numerous disadvantages of prior approaches. No prior accelerometer known to inventor has provided accurate measurement of the three orthogonal force components without the need for complex circuitry and calculations. Further, no known prior accelerometers have been fabricated from a single substrate wherein the accelerometer is directly responsive to the three orthogonal force components.